There is an inexorable trend toward storing and sending immutable data. We need immutability to coordinate at a distance, and we can afford immutability as storage gets cheaper. This article is an amuse-bouche sampling the repeated patterns of computing that leverage immutability. Climbing up and down the compute stack really does yield a sense of déjà vu all over again.

It wasn't that long ago that computation was expensive, disk storage was expensive, DRAM (dynamic random access memory) was expensive, but coordination with latches was cheap. Now all these have changed using cheap computation (with many-core), cheap commodity disks, and cheap DRAM and SSDs (solid-state drives), while coordination with latches has become harder because latch latency loses lots of instruction opportunities. Keeping immutable copies of lots of data is now affordable, and one payoff is reduced coordination challenges.